WASHINGTON — Representative James P. McGovern, the Worcester Democrat, is a steadfast liberal who has little in common with the conservative congressman a few doors down, Representative Phil Gingrey, a Tea Party-backed Republican from Georgia.

But the two are finding rare agreement in their shared reservations about a US attack on Syria.

As Congress marches toward a vote next week on authorizing strikes on Syria, the congressmen are withholding support, making them part of an emerging odd coalition that could determine the outcome of President Obama’s plan for targeted strikes in Syria.

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It’s not often that the conservative limited-government movement finds common ground with the liberal antiwar movement — especially in this era of partisan gridlock.

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Opponents of the Syria strike gave varying reasons for their stand. A growing number of Republicans have expressed skepticism about international engagement, and worry about the cost of military intervention. The Tea Party movement has been built by activists who crave a limited role of government.

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McGovern noted that many in the GOP have spoken out in favor of his annual amendment to withdraw troops from Afghanistan, an effort which passed in June with support from 120 Republicans.

On the Democratic side, there are antiwar activists who trace their concerns to Vietnam. Others built their careers on opposition to the Iraq War or felt burned by authorizing that war on what turned out to be faulty intelligence reports.

The same sentiment is reflected among special interest groups that are usually at odds. The libertarian Cato Institute has opposed the Syria resolution, as has Heritage Action, a political arm of the conservative Heritage Foundation. On the left, several groups have said they are opposed, including MoveOn.org, a group that rose to prominence opposing the Iraq War.

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House Speaker John Boehner and House minority leader Nancy Pelosi support Obama’s plan, a rare bipartisan pairing the White House hoped would be key to winning the vote.

But passage remains uncertain as liberals and conservatives show their willingness to unite.

“You may be an adversary when it comes to protecting . . . food stamps,” said McGovern, who has a poster in his office honoring Massachusetts soldiers who died in battle. “And you may be my ally in stopping some of these unnecessary wars.”

McGovern and Gingrey, like many members, said they hadn’t made a final decision, underscoring how important the debate about the Syria resolution — and reaction from constituents — will be in the days leading up to next week’s vote.

Gingrey said earlier in the week that, “While the use of chemical weapons is intolerable, the United States must not get mired down in the Syrian civil war.” His press secretary said Wednesday that Gingrey “has many concerns regarding US involvement in Syria,” but had yet to make a final decision.

‘If the United States doesn’t hold Assad accountable for this, it’s guaranteed he’ll do this again. Guaranteed.’ —Secretary of State John Kerry, During congressional testimony

Other conservatives have already made up their minds. Representative Ted Poe, a Texas Republican, said Wednesday that he was a definite “no” vote as he left a Foreign Affairs Committee briefing with Secretary of State John F. Kerry. Poe said he had spoken with people from both political parties and believes many Americans are concerned about an escalation of US involvement in another country’s civil war.

Although cooperation between conservatives and liberals is not common, it is not unheard of on Capitol Hill. In July, Representative Justin Amash, a Michigan Republican, pushed for a vote that would defund national security surveillance programs.

The provision failed narrowly – by a 205-to-217 vote – but drew a surprising amount of bipartisan support. That group is providing the baseline for those who may see Syria through a similar lens, and Amash has been similarly critical of some establishment Republicans for holding what he thinks is an out-of-date world view espoused by President George W. Bush’s administration.

“GWB-era foreign policy is nearly extinct among GOP grassroots,” he wrote on his Twitter account Monday. “Some Rs in DC either didn’t get the memo or haven’t been home in a while.”

Representative Alan Grayson, a liberal Democrat from Florida who was one of the most vociferous proponents of universal health care, is one of the most outspoken opponents.

“We are not the world’s policeman, nor its judge and jury,” Grayson wrote in an online petition that has been signed by nearly 35,000 people.

Everyone in the Massachusetts delegation, except for Representative Joseph P. Kennedy III of Brookline, voted to defund the NSA programs. And many in the all-Democratic delegation are also among those expressing the most skepticism about the Syria resolution.

Several Massachusetts delegation members have said they are leaning against the Syria strikes, including McGovern; Stephen F. Lynch, of South Boston; and Michael E. Capuano, of Somerville. Kennedy said he is undecided. Senator Edward J. Markey voted “present” on Wednesday on the resolution authorizing a Syria strike that was approved by the Senate Foreign Relations Committee in a 10-7 vote.

Although Boehner and Pelosi say they support Obama’s plan, they do not intend to lobby their members to vote for it. They are leaving that job to the White House, which has launched an intense push for votes.

“This is not the time for arm-chair isolationism,” Kerry said this week during congressional testimony. “This is not the time to be spectators to slaughter.”

Senator Rand Paul, a Kentucky Republican who has been a leading voice of opposition, had one of the most pointed exchanges with Kerry on Tuesday, challenging the administration’s case that the strikes on Syria would make the world safer.

“There are all kinds of unknowns that I can’t tell you absolutely the answer and neither can you,” Paul said. “But I think there’s a reasonable argument that the world will be less stable because of this. And that it may not deter any chemical weapons attack.”

“I haven’t had one person come up to me and say they’re for this war. Not one,” Paul added. “We get calls by the thousands. Nobody is calling in support of this war.”

Kerry bristled in his response, and turned his answer into a question.

“Let me just make it very clear to you,” he said. “You ask these questions, ‘Is this or that more likely to happen?’ If the United States doesn’t do this, senator, is it more or less likely [Syrian President Bashar] Assad does this again?”

“I think it’s unknown,” Paul said.

“Senator, it’s not unknown,” Kerry responded. “If the United States doesn’t hold Assad accountable for this, it’s guaranteed he’ll do this again. Guaranteed.”